Antiferromagnet-ferromagnet transitions in cobaltites

Three series of oxygen-deficient cobaltites, La0.5Ba0.5CoO3−δ, LnBaCo2O5.5 and Sr2YCo4O10.5, have been studied. La0.5Ba0.5CoO3 is an insulating ferromagnet whereas La0.5Ba0.5CoO2.75 is a pure antiferromagnet in which the oxygen vacancies are disordered. The ordering of the oxygen vacancies leads to the appearance of a ferromagnetic component, apparently because of the formation of a noncollinear magnetic structure. The antiferromagnet-“ferromagnet” transition is accompanied by a giant magnetoresistance. It is suggested that in the ferromagnetic oxidized compounds, the Co3+ and Co4+ ions adopt intermediate spin states, whereas in the antiferromagnetic (Co4+-free) compositions, the Co3+ ions have a high-spin state (CoO5 pyramids) with a dominant low-spin state (CoO6 octahedra). In both the ferromagnetic and antiferromagnetic compounds, superexchange via oxygen plays an essential role in determining the magnetic properties.